1. Field of the Invention
The present invention generally relates to the reduction of EMI in systems using a Pulse Width Modulated (PWM) signal. A PWM signal is defined as a periodic signal in which the duty cycle or pulse width may vary from one cycle to another. PWM signals are used in numerous applications such as motor drive control, error sensing and power supply design.
2. Description of Related Art
EMI (electromagnetic interference) is the generation of undesired radiation energy which has the potential of interfering with the operation of electronic systems. Regulatory bodies exist in most countries to set standards and guidelines for EMI and these bodies also monitor emissions by industrial, commercial and domestic systems to ensure compliance.
Numerous articles in the prior art describe methods of generating periodic frequency modulated signals. This patent specifically addresses the area of reconstruction of a PWM signal, which carries certain information content embedded within the signal (and represented by the pulse width or duty cycle of the pulse) while adding frequency modulation. Thus the new FM modulated PWM signal retains a significant percentage of the original information, which may be recovered by the system and used as if the FM component had never been added. This may be achieved in many ways and this patent illustrates four embodiments but is not restricted to these alone. The percentage of error in the reconstructed signal is directly related to the ratio between the minimum pulse width (smallest duty cycle) of the PWM and the carrier frequency used to perform the modulation. This relationship is described in the following specification.
Due to the nature of the PWM signal it is not a trivial task to frequency modulate the signal and retain at least a significant portion of the original information present in the PWM signal. This is because the information present in the PWM signal is contained in the pulse width of the pulse train on a per cycle basis. Conventional and widely accepted methods of frequency modulation are capable of generating a signal which has the same period as the original signal but cannot replicate the varying pulse width on a per cycle basis. As an illustration, a well known FM method is to use a phase locked loop (PLL) to directly or indirectly receive the signal to be modulated and then modulate the loop in one of many methods. The final modulated signal is derived from the frequency generating element of the PLL (the voltage controlled oscillator or current controlled oscillator) and the final duty cycle is usually constant and a reflection of this PLL element rather than of the original incoming signal having varying duty cycle.
U.S. Pat. No. 6,366,067 describes a system for reducing EMI in a power supply. It uses a PWM to generate a switching signal at a modulated frequency. It does not use the term “spread spectrum,” but it is modulated over a “selected spread of frequencies.” It is significant that there is no disclosure regarding the means for applying frequency modulation to the PWM signal.
It appears that attempts to apply spread spectrum techniques to PWM tend to alter the pulse width characteristics on a per cycle basis, due to the shifting frequency of the pulse signal. This alteration may introduce unwanted signal or voltage levels in the output, and an arrangement to avoid this problem would be a welcome innovation in the art.